Method for producing lithium vanadium polyanion powders for batteries

Abstract

This invention relates to a process for producing an improved cathode powder for making lithium ion batteries wherein the powder comprises lithium, vanadium and a polyanion. The process includes forming a solution-suspension of the precursors, which include vanadium pentoxide, with a reducing agent, a solvent, and a carbon-residue-forming material. The reducing agent causes the vanadium in vanadium pentoxide to reduce from V5+ to V3+. The solution-suspension is heated in an inert environment to drive the synthesis of the LVP (Li3V2(PO4)3) such that the carbon-residue-forming material is also oxidized to precipitate in and on the LVP forming carbon-containing LVP or CCLVP. The liquids are separated from the solids and the dry powder is heated to a second higher temperature to drive the crystallization of the product. The resulting product retains a small particle size, includes carbon in the LVP for conductivity and is created with very low cost precursors and avoids the need for milling or other processing to reduce the product to a particle size suitable for use in batteries. It also does not require the addition of carbon black, graphite or other form of carbon to provide the conductivity required for use in batteries.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of U.S. provisional patent application Ser. No. 60 / 933,866, filed Jun. 8, 2007.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT[0002]NoneFIELD OF THE INVENTION[0003]This invention relates to materials for use in the positive electrode of lithium-ion batteries and processes for making such materials.BACKGROUND OF THE INVENTION[0004]Lithium-ion batteries are recognized and valued for high efficiency, energy density, high cell voltage and long shelf life and have been in commercial use since the early 1990's. As always though, there is a desire to make better batteries for less cost.[0005]A key component of current lithium-ion batteries is a lithium transition-metal polyanionic powder that is provided as the active material on the metal plates at the positive electrode. Iron, cobalt, manganese, and nickel transition-metal powders have been used and other transition metals have been considere...

AI Technical Summary

Benefits of technology

[0043]One particular advantage of the present invention is that including the CRFM with the other precursors at appropriate ratios results in two desired reactions occurring almost simultaneously. The reducing agent reduces the vanadium from the V5+ to the V3+ valence state and the vanadium oxidizes the CRFM, causing it to become less soluble and to precipitate on and probably within the resulting LVP particles. This small amount of elemental carbon provides improved electrical conductivity in the LVP that is highly desired for use in batteries. As such, the LVP is described to be carbon-containing or CCLVP.

[0044]The CCLVP, as yet, does not have the degree of crystallinity that is desired for the final product. The temperature of the CCLVP powder is increased to a temperature higher than 300° C. in an inert atmosphere. The heating treatment temperature should be between 400 and 1000° C., preferably between 500 and 900° C., more preferably between 650 and 850° C. The resulting mixture remains as a loose powder. The heating at this step provides the necessary condition to form the desired crystalline structure for the final product.

[0045]It has been found that if the carbon-content of the resulting particles is not greater than 0.1 wt %, then the CCLVP powder does not have sufficient electrical conductivity to perform in a battery without some additional materials. Graphite or carbon black may be used as is well known in the art. More preferably a carbon coating as described in U.S. Pat. No. 7,323,120 and also in PCT Published Application Number WO 2007 / 082217 may be applied to the low carbon content powder (<0.1 wt %) to provide the electrical conductivity. Essentially, this additional coating process comprises applying the coating on the powder while the powder is suspended in a solution of CRFM using a selective precipitation method. The CCLVP with the CRFM coating is then heat treated to convert the CRFM to carbon and to bond the carbon coating firmly to the CCLVP particle. The heating temperature at this step should be between 500 and 1000° C., preferably between 600 and 900° C., more preferably between 700 and 900° C. The amount of carbon on and in the CCLVP is preferably above 0.5 wt % and up to about 10 wt %, but between 0.5 wt % to about 5 wt % is preferred and between 1 wt % and 3 wt % is most preferred.

[0046]Although carbon coating has been discussed, the preferred embodiment of the present invention is to create CCLVP having the preferred carbon content without having to provide additional carbon through additional steps. As noted above, the preferred carbon content is between 0.5 wt % and 10 wt %, preferably between 0.5 wt % and 5 wt %, and between 1 wt % and 3 wt % being most preferred.

[0047]Turning now to focus on several variations or embodiments of the inventive process, FIG. 2 indicates that the precursors are five valence vanadium, lithium carbonate, phosphoric acid and NMP. The precursors are heated up to a temperature between about 200° C. and about 300° C. such that the NMP reduces the five valence vanadium and synthesizes the LVP as a precipitate. The liquid is recycled through a process that eliminates water and light byproducts and the solid is pass on to an intermediate heat treat up to a temperature between about 350° C. and about 650° C. The liquid-solid separation is accomplished by mechanical separation such as vacuum filtration, centrifugal separation or other known means. After the intermediate heat treatment to create a more stable particle size and shape in the LVP, a pitch coating step is accomplished by selective precipitation, as described in U.S. Pat. No. 7,323,120. Briefly, the CRFM is dissolved in a solvent and combined with the LVP. The carbon is selectively precipitated on the particles at about 1% to 10% by weight. The coated LVP particles are then separated from the solvent and the particles are subjected to a third heat treatment to carbonize the carbon coating. The carbon coating may be first stabilized by a heat treatment process and then carbonized at a higher temperature or may be carbonized without being first stabilized.

[0048]In FIG. 3, the process is similar to that shown in FIG. 2 except that the intermediate heating step is omitted. The intermediate heating step is preferred, but is not necessary to practice the invention and produce CCLVP powder.

Problems solved by technology

Cobalt has high performance but has proven to be unsafe because of the potential for explosion during recharging.

Iron is attractive because of its low cost, but does not provide the energy density of other transition-metals such as cobalt and nickel.

Vanadium has been proposed, but has yet to be used commercially, probably because of the higher expense and limited success in obtaining any advantage over other, more developed systems.

However, there are several problems with each of these methods.

The major problems include a) agglomeration of particles, b) incomplete reactions, c) the existence or presence of undesirable components within the starting materials and their subsequent presence in the final products, d) poor electrochemical properties of the resulting materials, and e) the requirement for expensive precursors and / or complicated processes.

Since the particles of the active materials may be relatively large and / or the sizes may be non-uniform, optimum conditions of surface to surface contact between particles is often not well achieved.

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